The present invention relates to a process for preparing a developer for electrostatic images, particularly for use in an electrostatic high speed printer.
A known developer for electrostatic images comprises a mixture of smaller resinous toner particles and larger carrier beads made of iron or glass. The toner particles are held on the surface of the carrier beads by electrostatic force, which develops from the triboelectric charging of the toner particles and the carrier in opposite polarities due to contact therebetween. When the developer is moved into contact with the latent electrostatic images formed on a photoreceptor, the toner particles are attracted to the latent images, and, thus, the images are developed. The developed images are transferred onto plain paper and fixed thereon by heating.
Generally, such a dry developer has difficult-to-solve problems such as toner filming on the carrier beads and throwing-off of the toner particles during agitation of the developer in a developing station. Here, the term "throwing-off" means that the toner particles are thrown out from the developing station and scattered during the rotation of the magnetic roll of the developing station by moving air, centrifugal force, or mechanical shock.
Toner filming occurs due to a number of collisions between the toner particles and the carrier during recycling of the carrier. The attendant mechanical friction causes the toner material to partly melt and stick on the surface of the carrier beads, thus forming a film of toner thereon. Toner filming impairs the normal triboelectric charging of the toner particles in the developer, because the normal toner-carrier triboelectric charging is partly replaced by a toner-toner relationship. The improperly charged toner particles can be deposited on the non-image areas, whereby the quality of the printed mater is impaired, because the non-image areas possess an unacceptable darkness of background.
When the toner filming grows to a certain degree, the entire developer must be replaced, thereby increasing the cost of operation of a development apparatus. Furthermore, the replacement is time consuming. This is especially significant in a high speed printer.
The throwing-off problem occurs due to weak electrostatic charging between the toner particles and the carrier beads. When the developer material is vigorously agitated in the developing station, the toner particles are attracted to and adhered on the carrier beads. However, some of the toner particles are apt to separate from the carrier and fly up to deposit on non-image areas of the photoreceptor, and also, on the interior of the printer. As a result, fogging of the images occurs and the performance of the optical system, including the corotron, is impaired. Consequently, the electrostatic charging must be large enough to avoid throwing-off of the toner particles.
It is well known that carrier beads can be coated with a resin so as to reduce the formation of toner filming. However, the resin must be easily and uniformly coated on the surface of the carrier beads and firmly fixed thereon.
The coated carrier beads and the toner particles are required to exhibit at least the following features during operation:
(A) The carrier beads must effect the necessary triboelectric charging of toner particles and a sufficient number of toner particles must be held on the coated surface of the carrier beads to exhibit a high optical density of the images and, also, to reduce the throwing-off of the toner particles so as not to cause fogging of the images.
(B) The coating resin must not peel off from the carrier beads and must be resistant to abrasion. Even if the coating chips, flakes, or spalls, the resulting fine powder must not cause fogging of the images and corrosion of the developing station, and toxic materials must not be released.
(C) The resinous material of the toner particles must not stick to the coating resin during agitation, that is, must reduce the formation of undesirable toner filming which impairs the normal triboelectric charging. In order to improve the resolution, toner particles must not blot on the paper during fusing of the binder of the toner.
(D) The resinous material of a toner particle must not stick to that of other toner particles, that is, must reduce blocking of the toner particles.
Prior art developers for use in an electrostatic high speed printer did not satisfy all of features (A) through (D) above, although several patents disclose methods of improving the behavior of either the toner or the carrier. For example, carrier beads are coated with a fluorine-containing polymer and a modifying material which are physically mixed with each other. It is necessary in order to obtain a uniform coating that a diluted formulation of the polymer, which includes chromium oxide, be sprayed onto the carrier beads suspended in a cyclic fluidized bed tower. The process is complicated.
Also, in the prior art, the ratio of toner weight to carrier weight has been generally used to control the ratio of toner charge to toner mass. However, we found that such a weight to weight ratio has certain disadvantages when we define in detail the ratio of toner charge to toner mass. From a practical viewpoint, both the toner and carrier may exhibit various densities and shapes, as they are produced from various materials. For example, new techniques using flash fusible toners, resin-coated carriers, and carriers having nonsphericity have been developed in electrophotography.